I've been doing it for about 5 years now. Good news: My profit on a bike is usually between $100 to $140 per bike. Bad news: Finding bikes that I can make that kind of profit on is rare. I've been burned by buying a cheap bike in bad condition. Losing money due to a surprise happens. When it does, you lose. If it wasn't for cheap swapmeet parts & tires, the profit margin would be disaster.
More bad news: My return per hour spent fixing up a bike is about $6-$8. I have to assume I'll be running into some of my buyers eventually. I want them to greet me fondly. That's the standard I try to deliver. I've spent about $150 on special tools.
I enjoy the work. More entertaining than cable TV. Made a few thousand bucks over the years. It's not the road to riches.